This invention relates to a focus adjustment system for an optical disk apparatus.
An optical disk apparatus is a device, such as a compact disk player, video disk player, optical image file, or an optical disk drive in a computer system, that focuses a light beam through an objective lens onto a rotating disk in order to reproduce information from, and in some cases to write information on, the disk. Focusing of the light beam must be accurately adjusted and controlled to keep the objective lens at the optimum distance from the surface of the disk, which may not rotate in a perfectly level plane.
Focus is generally controlled as follows: a focus error signal representing the distance between the objective lens and disk is detected; the focus error signal is compared with a reference signal corresponding to the in-focus point; and the objective lens is moved so as to reduced the difference between the focus-error signal and reference signal to zero. Many focus adjustment systems, differing in the ways in which the focus-error signal is detected, are known from the prior art.
A problem with the prior-art systems is that the reference signal must be precisely adjusted when the optical disk apparatus is manufactured. This requires, for example, precise alignment of an optical system in order to focus light reflected from the disk onto the exact center of a split photodetector. It also requires the objective lens to be moved in relation to the disk by special test equipment to find in-focus point, and the reference signal to be adjusted precisely to the value of the focus-error signal at the in-focus position.
These alignments and adjustments are difficult to perform accurately, and tend to add to the time and cost of the manufacturing process. Furthermore, even if the focus adjustment system is precisely adjusted in the factory, there is no guarantee that it will remain in precise adjustment during the operating life of the optical disk apparatus. For example, mechanical stress may cause the light reflected from the disk to migrate to an off-center position on the photodetector, or aging of electronic components may cause the reference signal to vary, the result in either case being loss of focusing accuracy.